Use the fallthrough statement in switch loops

Clang does not seem to work with 'fall through' in comments.

Change-Id: Idcbe373be33ef7247548f856bfaba7ceb7f749b5
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/51498
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>

src/superio/nuvoton/nct5104d/superio.c

@@ -183,7 +183,7 @@ static void nct5104d_init(struct device *dev)
 	case NCT5104D_GPIO0:
 	case NCT5104D_GPIO1:
 		route_pins_to_uart(dev, false);
-		/* FALLTHROUGH */
+		__fallthrough;
 	case NCT5104D_GPIO6:
 		if (conf->reset_gpios)
 			reset_gpio_default_in(dev);